When functioning normally, the heart produces rhythmic contractions and is capable of pumping blood throughout the body. However, due to disease or injury, the heart rhythm may become irregular resulting in diminished blood circulation. Arrhythmia is a general term used to describe heart rhythm irregularities arising from a variety of physical conditions and disease processes. Cardiac rhythm management systems, such as implantable pacemakers and cardiac defibrillators, have been used as an effective treatment for patients with serious arrhythmias. These systems typically comprise circuitry to sense electrical signals from the heart and a pulse generator for delivering electrical stimulation pulses to the heart. Leads extending into the patient's heart are connected to electrodes that contact the myocardium for sensing the heart's electrical signals and for delivering stimulation pulses to the heart in accordance with various therapies for treating the arrhythmias.
Cardiac rhythm management systems operate to stimulate the heart tissue adjacent to the electrodes to produce a contraction of the tissue. Pacemakers are cardiac rhythm management systems that deliver a series of low energy pace pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency. Pace pulses may be intermittent or continuous, depending on the needs of the patient. There exist a number of categories of pacemaker devices, with various modes for sensing and pacing one or more heart chambers.
When a pace pulse produces a contractile response in heart tissue, the contractile response is typically referred to as capture, and the electrical cardiac signal corresponding to capture is denoted the evoked response. Superimposed with the evoked response may be a pacing artifact signal including, for example, the signal associated with post pace residual polarization. The magnitude of the pacing artifact signal may be affected by a variety of factors including lead polarization, after potential from the pace pulse, lead impedance, patient impedance, pace pulse width, and pace pulse amplitude, for example.
A pace pulse must exceed a minimum energy value, or capture threshold, to produce a contraction. It is desirable for a pace pulse to have sufficient energy to stimulate capture of the heart without expending energy significantly in excess of the capture threshold. Thus, accurate determination of the capture threshold is required for efficient pace energy management. If the pace pulse energy is too low, the pace pulses may not reliably produce a contractile response in the heart resulting in ineffective pacing. If the pace pulse energy is too high, the result may be patient discomfort as well as shorter battery life.
Capture detection allows the cardiac rhythm management system to adjust the energy level of pace pulses to correspond to the optimum energy expenditure that reliably produces a contraction. Further, capture detection allows the cardiac rhythm management system to initiate a back-up pulse at a higher energy level whenever a pace pulse does not produce a contraction.
A fusion beat is a cardiac contraction that occurs when two intrinsic cardiac depolarizations of a particular chamber, but from separate initiation sites, merge. When the heart is being paced, a fusion beat may occur when an intrinsic cardiac depolarization of a particular chamber merges with a pacer output pulse within that chamber. Fusion beats, as seen on electrocardiographic recordings, exhibit various morphologies. The merging depolarizations of a fusion beat do not contribute evenly to the total depolarization.
Pseudofusion occurs when a pacer output pulse artifact is superimposed upon a spontaneous P wave during atrial pacing, or upon a spontaneous QRS complex during ventricular pacing. In pseudofusion, the pacing stimulus is ineffective because the tissue around the electrode has already spontaneously depolarized and is in its refractory period.
During normal pacing, the presence of fusion and pseudofusion beats may be of little consequence except for wasted energy due to the generation of unnecessary pace pulses. However, detection of fusion and pseudofusion beats may be required during an automatic capture or threshold determination procedures. Fusion and pseudofusion beats may cause false detection of capture and may lead to erroneous capture threshold values.
Capture may be verified by detecting if a cardiac signal following a pace pulse indicates an evoked response. However, the evoked response must be discerned from the superimposed post pace residual polarization, denoted herein as a pacing artifact. In addition, fusion or pseudofusion beats may further obscure evidence of capture. It is desirable to detect the evoked response and thereby verify capture so that an effective pace pulse energy may be chosen and appropriate back up pacing delivered. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading the present specification, there is a need in the art for a method and device that reliably and accurately detects capture in a patient's heart by sensing an evoked response in the presence of the post pace residual polarization and possible fusion or pseudofusion beats. There exists a further need for such an approach that is adaptive and accommodates changes in the patient's capture threshold over time. The present invention fulfills these and other needs.